Disk drives are capable of storing large amounts of digital data in a relatively small area. Disk drives store information on one or more recording media, which conventionally take the form of circular storage disks (e.g. media) having a plurality of concentric circular recording tracks. A typical disk drive has one or more disks for storing information. This information is written to and read from the disks using read/write heads mounted on actuator arms that are moved from track to track across the surfaces of the disks by an actuator mechanism.
Generally, the disks are mounted on a spindle that is turned by a spindle motor to pass the surfaces of the disks under the read/write heads. The spindle motor generally includes a shaft mounted on a base plate and a hub, to which the spindle is attached, having a sleeve into which the shaft is inserted. Permanent magnets attached to the hub interact with a stator winding on the base plate to rotate the hub relative to the shaft. In order to facilitate rotation, one or more bearings are usually disposed between the hub and the shaft.
Over the years, storage density has tended to increase, and the size of the storage system has tended to decrease. This trend has lead to greater precision and lower tolerance in the manufacturing and operating of magnetic storage disks.
The bearing assembly that supports the storage disk is of importance. One bearing design is a fluid dynamic bearing. In a fluid dynamic bearing, a lubricating fluid such as air or liquid provides a bearing surface between a fixed member of the housing and a rotating member of the disk hub. In addition to air, typical lubricants include gas, oil or other fluids. The relatively rotating members comprise fluid dynamic grooves formed on the members themselves. Fluid dynamic bearings spread the bearing surface over a large surface area, as opposed to a ball bearing assembly, which comprises a series of point interfaces. This bearing surface distribution is desirable because the increased bearing surface reduces wobble or run-out between the rotating and fixed members. Further, the use of fluid in the interface area imparts damping effects to the bearing, which helps to reduce non-repeatable run-out. Thus, fluid dynamic bearings are an advantageous bearing system.
Many current fluid dynamic bearing motor designs used in small form factor drives—that is, drives with stringent axial height constraints—suffer from insufficient angular stiffness due to the limited journal space available for journal bearing span. It is well known in the art that angular stiffness is a function of linear stiffness (i.e., radial stiffness in the journal bearing and axial stiffness in the thrust bearing) times a moment arm length (i.e., journal bearing span and thrust bearing diameter). Angular stiffness shortcomings therefore have traditionally been countered by employing a large diameter thrust bearing (i.e., increasing the moment arm length—or thrust bearing diameter—for axial stiffness) to augment the total bearing angular stiffness in cases where increasing the journal span is not possible. However, increasingly stringent power requirements in small disk drives make this option less efficient from a power standpoint. More specifically, large diameter thrust bearings consume relatively large amounts of power because the drag of a thrust bearing is proportional to the 4th power of the bearing's radius, and the bearing fluid employed in those thrust bearings oftentimes is a liquid having a relatively high viscosity.
Thus, there is a need in the art for an efficient small form factor fluid dynamic bearing motor having improved angular stiffness.